A number of different types of packages for integrated circuits (ICs) have been developed over the years. The term “package” in this context means the enclosure or container that houses and protects the actual microelectronic circuitry. An IC package has external contacts or terminals that are internally connected to the various terminals of the IC itself. During assembly, the external IC package contacts are connected in some way to properly aligned contacts on a circuit board.
The IC contact connection can be done in a number of different ways. One way is through use of sockets in which the IC package mounts. Another, and the one with which this invention is concerned, is by directly bonding the IC package to terminals on the circuit board. The bonding process must provide secure mechanical attachment and good electrical connection between each package terminal and the circuit board contacts. Soldering is one common type of bonding used, but other techniques such as ultrasonic bonding are also used.
Solder bonding is often done by forming solder-coated contacts on the IC. The circuit board on which the IC is mounted has a set of contacts designed to align with the IC package contacts. The IC is then temporarily held with all of its contacts positioned against the corresponding circuit board contacts. By heating the solder on the contacts, for example with an infrared beam, the IC is electrically and mechanically attached to the circuit board.
FIG. 1 shows an IC package 10 called a “ball grid array” package that is frequently used now for solder bonding. In ball grid array packaging, an orthogonal grid of solder ball contacts 14 is formed on a flat surface 16 of the IC package exterior 12. The individual solder contacts forming grid 14 may be spaced by approximately 1 mm. from their orthogonal neighbors with anywhere from 5 to 50 contacts on one edge of the grid. Referring to FIG. 3, package 10 has a top surface 11 opposite the side carrying the contacts 14.
The package 10 is attached to the circuit board by pressing the solder ball contacts 14 against the corresponding circuit board contacts. The opposite side of the circuit board is then heated to a temperature sufficient to melt the solder contacts 14 on the IC package but not so hot that either the IC or the circuit board itself is damaged. The individual solder balls bond to the adjacent circuit board contacts, electrically and mechanically attaching IC package 10 to the circuit board.
As is well known, testing of individual ICs is an important part of IC and circuit board manufacturing. Even though IC manufacturing consistently produces a relatively low percentage of defective parts, it is usual to test most ICs and other components before installing. Ball grid array packages and most other types of ICs as well are for all practical purposes permanently mounted once soldered to a circuit board. Since it is difficult or impossible to replace the defective part, installing a defective IC or other component usually ruins the entire circuit board. For this reason it is usually cost-effective to test ICs before soldering them permanently onto the circuit board.
This testing can be done by both manual and automated test equipment. Typically, tests are carried out by mounting the IC in a test fixture having contacts that temporarily connect the IC package contacts to test equipment without soldering. Such an arrangement is shown in U.S. Pat. No. 5,360,348. The test equipment applies test signals and power to the IC through the contacts in the test fixture and receives output signals from the IC through the test fixture contacts. The test equipment can then check in the conventional way that specific sets of input signals to the IC cause the IC to provide predetermined output signals.
Test fixtures are often used for production testing of large numbers of IC devices. Extended life contacts are important in such test fixtures because the cost of the socket can be amortized over the production of a large number of IC devices. Extended life sockets also reduce down time required to maintain or replace the sockets.
When testing an IC device, good electrical contact to the IC leads is important for determining how well an IC functions. Measurements while testing an IC device ideally reflect only the part performance. That is, the test environment should not affect the outcome of the test procedure. I find that the contacts in the test fixture can on occasion affect measurements of the IC performance and even the actual performance of the IC itself.
Characteristics of a test fixture that determines its ability to effectively test IC performance include electrical contact reliability, electrical performance, thermal effects, and mechanical performance. These factors should be taken into account when designing a text fixture.
Two main factors impact the ability of the socket to establish reliable contact. The test fixture contacts must penetrate any contaminants on the IC leads, and the IC mounting in the test fixture must deal with lack of lead coplanarity. Test fixture contacts should form low resistance electrical connections with corresponding contacts of the IC to be tested. One design achieves this result by providing a “wiping” action or connection in which the lead to be contacted slides past, and is gently “roughened” by the socket contact, thereby breaking through contaminants carried thereon.
As to the issue of lack of coplanarity, it is necessary that there be compliance or flexibility somewhere in the test system so that the IC contacts and the test fixture contacts conform in a way that creates good electrical connection between each pair of contacts. Furthermore, compliance is advantageous since IC package irregularities, including non-parallel conditions and thickness variations, are not uncommon.
An actuator is a device that provides the force for creating the desired electrical contact between the test socket contacts and the IC leads. Actuators exist that have compliant members depending therefrom. Such compliant members may comprise deformable or otherwise resilient elastomeric bumpers or tabs (see for example U.S. Pat. No. 5,926,027 wherein such compliant elements are adhesively affixed for device alignment and engagement). But such actuators do not always provide optimal mechanical performance. Furthermore, these actuator designs may not transfer to test fixtures for ball grid array IC packages.
I find that using standard actuators to press ball grid arrays against an array of test contacts is sometimes not successful. Apparently the electrical characteristics of the contacts, such as resistance, are not consistent from one contact to the next, and from one test operation to the next. These variations may occasionally lead to rejection of some IC's that are actually good. In other cases, the test procedure returns inconsistent or indeterminate results as contact resistances change during the test activity.
Thus there remains a need to provide a compliant actuator that reliably applies contact-creating forces to a device under test. Such an actuator should perform repeatedly and reliably in compensating for device package irregularities and non-coplanarity between the actuator and contactor, under temperature extremes and mechanical rigor.